Carbon fiber reinforced polymer (CFRP) composites are rapidly replacing metallic structural components in aerospace and automotive applications owing to their high strength to weight ratio and other excellent mechanical properties. Even though the general practice is to keep CFRP composites electrically isolated from aluminum alloys in aircraft structures, they can become connected when a layer of moisture condenses over their surfaces creating a galvanic couple. In such cases, the more noble carbon fibers of the composite material will act as the cathode and aid the reduction of dissolved oxygen. The less noble aluminum alloy will become the anode and get oxidized at an accelerated rate– so-called galvanic corrosion which can be catastrophic and is a significant challenge encountered in aerospace and automotive assets. Corrosion of the aluminum alloy can be inhibited by decreasing the rate of aluminum oxidation, decreasing the rate of cathodic reaction, or decreasing the rate of both. The cathodic reaction rate can be reduced by blocking the active sites of exposed carbon fibers for O2 chemisorption-the first step of oxygen reduction reaction (ORR). This is accomplished by chemically modifying the exposed carbon fiber surfaces of the composites. The first part of research described herein focuses on exploring the influence of chemical modification of carbon fiber epoxy composite edges with substituted phenyl diazonium adlayers on the electrochemical reduction of dissolved oxygen, how stable the modified surface is and how much the rate of galvanic corrosion of aluminum alloy is reduced during accelerated degradation tests. The electrochemically assisted and spontaneous formation of diazonium molecular adlayers on the exposed fibers of a carbon fiber reinforced polymer (CFRP) composites were investigated. The formation of stable adlayers was confirmed and adlayer coverage was determined by Raman spectroscopy and cyclic voltammetry. The influence of diazonium surface treatment on the ORR kinetics was assessed by measuring the voltammetric curves for dissolved oxygen reduction on unmodified and chemically modified composites in naturally aerated 0.5 M Na2SO4. Two accelerated degradation test paradigms were investigated: (i) neutral salt spray (ASTM B117) and (ii) thin layer mist spray. Galvanic corrosion on the aluminum alloy was evaluated by weight change measurements, visual observation with digital optical microscopy and scanning electron microscopy, and surface topographical analysis by digital optical microscopy, confocal laser microscopy, and optical profilometry. The stability of the adlayer after accelerated degradation tests was determined by Raman spectroscopy observing for the presence of the adlayer. Due to the growing demand for titanium alloys in the aviation sector and the high cost associated with conventional subtractive manufacturing methods for titanium components, there has been a significant focus on utilizing additive manufacturing (AM) techniques for fabricating titanium components. Fundamental research is needed to better understand how the fabrication parameters influence the material density, microhardness, microstructure, and electrochemical corrosion susceptibility of such AM titanium alloys. The second part of this dissertation work focuses on investigating the material and electrochemical properties of titanium alloys prepared by selective laser melting (SLM). The surface morphology and microstructure of the as-processed and surface-pretreated titanium alloy (Ti-5553) specimens were investigated employing scanning electron microscopy. The surface pretreatment involved abrading and polishing to reduce the surface roughness and smooth the surface texture. The electrochemical corrosion behavior of the as-processed and surface-pretreated titanium specimens were studied by performing open circuit potential and electrochemical impedance spectroscopy (EIS) measurements and recording potentiodynamic polarization curves.
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